1. Field of the Invention
This invention relates in general to safety circuits for detecting asymmetry in load currents and de-energizing the load circuit when asymmetry is detected, and more particularly, to a safety circuit that is useful with a triac to cut off load current flow through the triac when it detects asymmetrical load current through the triac.
2. Description of the Related Art
With the development of semiconductor switching devices it became evident that some safety means must be developed to safeguard both the switching device and the associated load from asymmetries in the load current that would otherwise damage or destroy both of these devices.
The need for the development of a safety means became especially important with the use of a class of discrete semiconductor switching devices classified as thyristors. The two most important subclassifications of thyristors for use in understanding the preferred embodiment of the invention disclosed and claimed herein, are the silicon reverse-controlled blocking thyristor or silicon controlled rectifier (SCR) as it is commonly known, and the bidirectional triode thyristor or triac as it is commonly known.
Thyristors are generally used as switching devices to deliver unidirectional load currents that do not require any control current once they are switched on. Normally, all that is required to switch them on is a quick triggering pulse to a control gate, after which, even if the triggering pulse is removed, the thyristor continues to conduct its load current. Typically, they cannot be turned off by means of a control current, so they cannot be controlled continuously, as a transistor can. In fact, they normally cease conduction only when the load current itself stops. It is this latter characteristic of a thyristor that can cause problems when a triac is used as a switching device in an alternating current circuit. Since the current goes to zero, shutting off the triac at the end of each half-cycle, the triac must be re-triggered every half cycle, there is the possibility that the triac will be triggered into conduction on one-half of the applied alternating current cycle and not on the other half of the cycle. The result of this malfunction would be to apply voltage with a significant direct current component to the system load. When the system load is, for example, a fluorescent lighting system, it is essentially inductive in its characteristics. Applying a significant direct current voltage component to an inductive load of this type can cause overcurrents and overheating. Where the lighting system ballasts do not incorporate thermal cut-out devices, the overheating may permanently damage the ballasts. The safety means of the present invention was developed to detect such half-cycle operation by a switching device such as a triac and to effectively cut off the power to the system load in the event of its occurrence.